Provision of water is a problem where rainfall is scarce, strongly seasonal, or where there are relatively small catchment areas and little natural local water storage. This is particularly true for remote locations such as oceanic islands and for coastal areas where the fresh water table is relatively shallow or not well developed because of subsurface geological conditions.
On oceanic islands, for example on Bermuda in the central Atlantic Ocean, water is provided mainly by rain catchment on most buildings, including virtually all private homes. Water is stored locally in cisterns into which the run-off flows directly. Because the water is derived from rainwater run-off from roofs, there is often both biological and sediment contamination. When rainfall is sparse, the collection of water is insufficient for demands and where populations are high, water rationing is common. Low rainfall also increases the level of pollution. In addition, where roofs are of such a size that not enough water can be captured directly from run-off for local use, for instance from roofs of factories on Saipan in the western Pacific, water must be provided from another source.
Similarly, the capture of rainwater and pumping of shallow wells for human use, including industrial purposes, in localities where there is a very thin fresh water groundwater layer on top of a saline-saturated substrate substantially degrades the environment on many of these islands and related localities. Capture of rainwater prevents it from recharging fragile groundwater systems.
Water companies in most of the United States plan water requirements based on an average water use for an individual in an urban environment of about 100 gallons per day. Where water supplies are restricted, such as in most of the Caribbean Islands for instance, 50 gallons per day represents average per capita total water use. In arid areas or where water infrastructure is poor, average per capita consumption (for all purposes) is commonly below 10 gallons per day, even when water supplies are normal. Natural water resources often do not meet local demands now, and population growth is increasing. Thus, new sources of fresh water for human consumption are required now.
In addition, relatively smaller amounts of high quality potable water are required from time to time where natural or man-made disasters render water and electrical infrastructure unusable, or in remote locations where distributed water supplies are required and no water infrastructure exists.